This invention is concerned with a method of treating kaolinitic clays, especially to improve the properties of such clays for use as ingredients for ceramic forming compositions, especially compositions which are to be used for preparing ceramic articles, eg whiteware articles such as tableware and the like.
Ceramic articles, eg tableware for use in the home and in the catering industry, are generally formed from a wet high solids composition which comprises a blend of various particulate ingredients which include kaolinitic clays, ie clays which contain the mineral kaolinite, such as kaolin or china clays and/or ball clays. Usually fluxing materials such as china stone, feldspar or nepheline syenite, and at least one silica-containing material, such as quartz or flint are also included in such compositions. If it is desired to produce articles of bone china, the composition will also contain a substantial proportion of ground, calcined animal bone, especially from cattle, or bone ash. The composition may also include minor proportions of other ingredients such as calcium carbonate, dolomite and talc. The proportions of the various ingredients used in the composition vary according to the properties required in the fired ceramic article. Many different types of ceramic tableware are roduced in various parts of the world, including, fine earthenware, semi-vitreous china, semi-vitreous porcelain, hotel china, household china, bone china, hard porcelain and stoneware.
Ceramic tableware articles are generally formed from the wet ceramic forming composition by a process which is based on the ancient technique of hand throwing on a potter""s wheel. The technique of free-hand throwing is still used for shaping individual art pieces, but when a large number of substantially identical articles are required to be formed, a degree of automatic mechanisation is incorporated into the process. In this latter case, a mould of a suitable material, for example plaster or a synthetic resin, may be employed fixed to a wheel which is capable of high speed rotation in a horizontal plane. A suitable amount of the ceramic composition is then introduced onto or into this mould. If the mould is substantially convex, and is used for shaping, for example, the inside of a plate or dish, the process is widely known as xe2x80x9cjiggeringxe2x80x9d. If, however, the mould is concave, and is used for shaping the outside of a cup or jug, the term xe2x80x9cjolleyingxe2x80x9d is often used. The second surface of the article, which is not in contact with the mould, is generally shaped by means of a profiling tool, most commonly of metal, which is brought into contact with this surface, whilst the article being shaped is rotated on the wheel. The shaping process has recently been rendered faster and more efficient through the introduction of roller head machines. In these machines the profiling tool is replaced by a heated rotating die, and both die and mould rotate continuously at appropriate speeds during the shaping of an article.
In order to perform satisfactorily in a shaping process, eg of the type described above, it is necessary for the ceramic forming composition to have sufficient plasticity to enable it to flow and deform under the action of compressive, tensile and shear stresses. The shaped article must also possess sufficient strength in its unfired or xe2x80x9cgreenxe2x80x9d state, to permit a certain amount of handling without loss of its integrity and shape. The green strength of a ceramic forming composition is generally determined by measuring the modulus of rupture (MOR) of dried extruded bars formed from the composition under certain standard conditions described later.
Some ceramic tableware is formed by a slip casting process. In this case the clays and other ingredients of the composition are mixed with a larger quantity of water, optionally with one or more additives, eg one or more dispersing agents, to form a fluid suspension, slurry or xe2x80x9cslipxe2x80x9d. The slip is poured into a porous mould where a shaped article is formed by a process which is similar to that by which a filter cake is formed in a filter press. Partial dewatering of the shaped article occurs as water passes from the composition through the porous walls of the mould, until the article is sufficiently formed, in a dry and firm state, to be removed from the mould.
A further shaping process used for forming articles of ceramic tableware is that of dust pressing. In this process a ceramic composition in the form of an aqueous suspension containing a relatively high concentration of solid material, together with one or more dispersing agents for the solid material, is subjected to spray drying to form substantially dry hollow microspheres of diameter of the order of about 0.1 mm. A charge containing an appropriate quantity of these microspheres is introduced into a suitable mould to which pressure is applied to compact the charge to form the desired ceramic article. Again, when articles are formed by dust pressing, it is necessary for the ceramic composition to possess sufficient green strength to enable the shaped article to be handled without undue risk of breakage.
Subsequent to the shaping process, whatever shaping method is used, the shaped body produced in its green state is dried before firing one or more times to a suitable temperature in a kiln, to produce a ceramic article of the type desired. Glazes and decoration may also be applied at this stage.
An object of this invention is to improve the properties of kaolinitic clay components of ceramic forming compositions in order to increase the strength of green shaped articles formed from the compositions.
According to the present invention there is provided a method of treating a kaolinitic clay which is intended for use as an ingredient in a ceramic forming composition which method comprises the steps of
(a) mixing with the kaolinitic clay from 0.1% to 15.0% by weight, based on the dry weight of the kaolinitic clay, of a smectite clay; and
(b) subjecting a moist mass in a plastic state of the mixture formed in step (a) to mechanical working under conditions such that there is dissipated in the moist plastic mass at least 5 kJ of energy per kilogram of the clay mixture on a dry weight basis.
The amount of energy dissipated in step (b) may be in the range of from 5 kJ to 300 kJ of energy per kilogram of the clay mixture on a dry weight basis.
The kaolinitic clay used in step (a) may already have been subjected to known preliminary processing or refining steps, eg steps selected from degritting, washing, magnetic separation of impurities and one or more particle size separation steps.
The moist plastic state mass treated by mechanical working in step (b) preferably contains between 20% and 30% by weight of water.
The mixture of clays produced in step (a) may have a water content which is suitable for use in step (b). Alternatively, the water content of the clay after production may be adjusted to provide a suitable moist mass in a plastic, workable state. The water content adjustment may be by addition of an aqueous liquid or by concentration, depending on the water content of the mixture produced in step (a).
Where the clay mixture produced in step (a) is in the form of a dry powder the required moisture content may be adjusted simply by addition of water and mixing.
Where the clay mixture produced in step (a) is in the form of a dilute slurry or suspension the required moisture content may be obtained by one or more known dewatering processes, eg filtering and/or pressing and/or partial drying and/or adding already dried material, ie using a dry feedback or dry-return supply loop from a subsequent dryer output.
The kaolinitic clay used in step (a) may comprise one or more kaolin clays of primary or secondary origin. Kaolinitic clays were formed in geological times by the weathering of the feldspar component of granite. Primary kaolin clays are those which are found at the site at which they were formed, and are generally present in a matrix of undecomposed granite which must be separated from the clay during the refining process for the clay. Secondary kaolin clays, which are alternatively known as sedimentary kaolin clays, are those which were flushed out in geological times from the granite matrix in which they were formed, and were deposited in an area remote from their site of formation, generally in a basin formed in the surrounding strata. Kaolin clays are generally found in association with relatively small proportions of impurities, such as mica, feldspar, quartz, titanium compounds and the like, and may also include a trace of smectite clays. The kaolinitic clay may alternatively comprise one or more ball clays, or a mixture of one or more ball clays with one or more kaolin clays. Ball clays are sedimentary clays which are very finely divided, in that they have a particle size distribution such that the particles predominantly have an equivalent spherical diameter smaller than 2 xcexcm. However, ball clays tend to have a higher proportion of impurities than kaolin clays, and to be less white in colour. The impurities present in ball clays may include significant proportions of fine silica, together with minor amounts of compounds of iron and titanium and also organic matter such as lignite.
Smectite clays are formed predominantly of smectite mineral particles which are sheet silicates with a high cation exchange capacity arising from charge imbalance due to substitutions within the crystal lattice. This charge imbalance is compensated by cations adsorbed from solution, known as exchangeable ions because they can easily be exchanged with ions of a different type. For most naturally occurring smectites, the exchangeable ion is a divalent cation, principally calcium, although a few smectites are found with a monovalent ion, principally sodium, as the exchangeable cation, notably smectites from Wyoming, USA.
In water, those smectites with divalent calcium cations disperse to a lesser degree than those with monovalent cations. This is due to the greater effect of the divalent cation in compressing the so-called electrostatic double layer around the particles that causes them to repel each other, compared with the monovalent cation.
Monovalent ion exchanged smectites are relatively easily dispersed in water to give individual plates or crystallites, whereas the divalent ion exchanged smectites tend only to disperse to xe2x80x9cpacketsxe2x80x9d or three or four crystallites. Monovalent ion exchanged smectites, especially sodium smectites, are generally more effective in user applications.
It is a relatively simple matter to convert a calcium smectite to a sodium smectite, by adding a small amount of a sodium ion containing solution, eg sodium carbonate, typically about 4% to 5% by weight. When dispersed in water, the exchangeable calcium ions are precipitated as calcium carbonate and the sodium ions become the exchangeable ions. The smectite is then said to be xe2x80x9csodium activatedxe2x80x9d. However, the term xe2x80x9cactivatedxe2x80x9d should be used with caution, as smectites can also be xe2x80x9cacid activatedxe2x80x9d for use in decolouring vegetable oils, which is an entirely different activation process.
In the method of the present invention, the smectite clay is preferably a montmorillonitic clay such as a bentonite, and preferably has a monovalent ion such as sodium as the predominant exchangeable cation. Such a clay can be prepared for example by activating a calcium bentonite with sodium carbonate. Other smectite clays such as hectorite, saponite and beidellite may be suitable for use in the method of the present invention.
The amount of the smectite clay mixed with the kaolinitic clay in the method of the invention is preferably in the range of from 0.5% to 7.0% by weight, based on the dry weight of the kaolinitic clay.
The smectite clay when added to the kaolinitic clay may be in the form of a powder or a slurry, ie an aqueous suspension. Likewise, the kaolinitic clay may be in powder or slurry form. The clays after being added together are preferably mixed thoroughly together for a period of time, eg at least 1 minute, preferably at least 2 minutes. Desirably, the mixture of the two clays is moist, eg contains at least 10% by weight water, in some cases from 10% to 90% by weight water, when the clays are being mixed together.
The clays may be mixed together in moist form in a mixing or compounding device. The individual clays may be added together on an inlet conveyer to such a device or conveyed separately for addition and mixing in the device.
In step (b) of the method of the invention, the water content of the treated moist plastic state mass is preferably from 23% to 28% by weight of water.
The mixture of clays may be treated by one or more additional procedures, in addition to any water content adjustment required, prior to step (b). For example, so called xe2x80x9ctramp ironxe2x80x9d or large pieces of iron may be removed prior to step (b) by a permanent magnet.
In step (b) of the method of the invention, the mechanical working may be exerted upon the plastic state mass by means of an extrusion device, such as an auger-type pug mill, a Z-blade mixer, an edge runner mill or a similar device known for working masses of particulate material in a moist plastic state. The device is preferably an auger-type pug mill, which is a known device, eg as described in GB1,194,866 the contents of which are incorporated by reference and is conveniently provided with known means for adjusting the size of the outlet nozzle(s) in order to control the throughput rate of material passing through the device, and the pressure built up inside the device, and thus the amount of energy dissipated in the plastic state clay mixture. The amount of energy dissipated in the plastic state mass is preferably in the range of from 10 kJ to 250 kJ, and most preferably from 20 kJ to 175 kJ, per kilogram of the clay mixture treated on a dry weight basis.
After treatment by steps (a) and (b) of the method of the invention, the resulting clay mixture product may optionally be further processed by one or more known refining processes.
The resulting product (with or without further processing) may be delivered to a user in wet slurry form or in dry powder form, eg by thermally drying the resulting product prior to delivery. The resulting product may be employed in the production of compositions to make ceramic articles in a known way, eg using one of the prior art methods described earlier.
The resulting product of the method according to the invention can, when extruded and dried in a standard manner, show an improved modulus of rupture and can also provide improved plasticity and higher solids castable compositions all of which are beneficial properties when the product is used in compositions for shaping to form ceramic articles. These benefits are demonstrated in the following Examples.
Embodiments of the present invention will now be described by way of example in the following Examples.